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Über dieses Buch

This book has evolved by processes of selection and expansion from its predecessor, Practical Scanning Electron Microscopy (PSEM), published by Plenum Press in 1975. The interaction of the authors with students at the Short Course on Scanning Electron Microscopy and X-Ray Microanalysis held annually at Lehigh University has helped greatly in developing this textbook. The material has been chosen to provide a student with a general introduction to the techniques of scanning electron microscopy and x-ray microanalysis suitable for application in such fields as biology, geology, solid state physics, and materials science. Following the format of PSEM, this book gives the student a basic knowledge of (1) the user-controlled functions of the electron optics of the scanning electron microscope and electron microprobe, (2) the characteristics of electron-beam-sample inter­ actions, (3) image formation and interpretation, (4) x-ray spectrometry, and (5) quantitative x-ray microanalysis. Each of these topics has been updated and in most cases expanded over the material presented in PSEM in order to give the reader sufficient coverage to understand these topics and apply the information in the laboratory. Throughout the text, we have attempted to emphasize practical aspects of the techniques, describing those instru­ ment parameters which the microscopist can and must manipulate to obtain optimum information from the specimen. Certain areas in particular have been expanded in response to their increasing importance in the SEM field. Thus energy-dispersive x-ray spectrometry, which has undergone a tremendous surge in growth, is treated in substantial detail.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
In our rapidly expanding technology, the scientist is required to observe, analyze, and correctly explain phenomena occurring on a micrometer (µ,m) or submicrometer scale. The scanning electron microscope and electron microprobe are two powerful instruments which permit the observation and characterization of heterogeneous organic and inorganic materials and surfaces on such a local scale. In both instruments, the area to be examined, or the microvolume to be analyzed, is irradiated with a finely focused electron beam, which may be static or swept in a raster across the surface of the specimen. The types of signals produced when the electron beam impinges on a specimen surface include secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies. These signals are obtained from specific emission volumes within the sample and can be used to examine many characteristics of the sample (composition, surface topography, crystallography, etc.).
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

2. Electron Optics

Abstract
The amount of current in a finely focused electron beam impinging on a specimen determines the magnitude of the signals (x-ray, secondary electrons, etc.) emitted, other parameters being equal. In addition, the size of the final probe determines the best possible resolution of the scanning electron microscope (SEM) and electron microprobe (EPMA) for many of the signals that are measured. Therefore the electron optical system in these instruments is designed so that the maximum possible current is obtained in the smallest possible electron probe. In order to use the instruments intelligently it is important to understand how the optical column is designed, how the various components of the optical system function, and how the final current and spot size are controlled. In this chapter we will discuss the various components of the electron optical system, develop the relationship between electron probe current and spot size, and discuss the factors which influence this relationship.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

3. Electron-Beam-Specimen Interactions

Abstract
The versatility of the scanning electron microscope for the study of solids is derived in large measure from the rich variety of interactions which the beam electrons undergo within the specimen. The interactions can be generally divided into two classes: (1) elastic events, which affect the trajectories of the beam electrons within the specimen without significantly altering the energy, and (2) inelastic events, which result in a transfer of energy to the solid, leading to the generation of secondary electrons, Auger electrons, characteristic and continuum x-rays, long-wavelength electromagnetic radiation in the visible, ultraviolet, and infrared regions, electron-hole pairs, lattice vibrations (phonons), and electron oscillations (plasmons). In principle, all of these interactions can be used to derive information about the nature of the specimen—shape, composition, crystal structure, electronic structure, internal electric or magnetic fields, etc. To obtain this information from the signals measured and the images recorded with the SEM, the microscopist needs a working knowledge of electronspecimen interactions, broadly qualitative and where possible, quantitative. This chapter does not provide an in-depth treatment of electron physics; rather, it attempts to provide an overview necessary for the analysis of SEM images and compositionally related signals. Additional detail will be given in later chapters devoted to qualitative and quantitative x-ray microanalysis.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

4. Image Formation in the Scanning Electron Microscope

Abstract
One of the most surprising aspects of scanning electron microscopy is the apparent ease with which images of rough objects can be interpreted by newcomers to the field, or even by laymen unfamiliar with the instrument. However, there is frequently more than meets the eye in SEM images, even images of simple objects, and to gain the maximum amount of information, it is necessary to develop skills in image interpretation. Moreover, to ensure that the image has been properly constructed and recorded in the first place, a good working knowledge of the entire image formation process is necessary. In this chapter, we will consider the major aspects of the SEM imaging process: (1) the basic scanning action used for the construction of an image; (2) the origin of the commonly encountered contrast mechanisms which arise from the electron-specimen interaction; (3) the characteristics of detectors for the various signals and their influence on the image; (4) signal quality and its effect on image quality; and (5) signal processing for the final display.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

5. X-Ray Spectral Measurement: WDS and EDS

Abstract
Chemical analysis in the scanning electron microscope and electron microprobe is performed by measuring the energy and intensity distribution of the x-ray signal generated by a focused electron beam. The subject of x-ray production has already been introduced in the chapter on electronbeam-specimen interactions (Chapter 3), which describes the mechanisms for both characteristic and continuum x-ray production. This chapter is concerned with the methods for detecting and measuring these signals as well as converting them into a useful form for qualitative and quantitative analysis.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

6. Qualitative X-Ray Analysis

Abstract
The first stage in the analysis of an unknown is the identification of the elements present, i.e., the qualitative analysis. Qualitative x-ray analysis is often regarded as straightforward, meriting little attention. The reader will find far more references to quantitative analysis than to qualitative analysis, which has been relatively neglected in the literature, with a few exceptions (e.g., Fiori and Newbury, 1978). It is clear that the accuracy of the final quantitative analysis is meaningless if the elemental constituents of a sample have been misidentified. As a general observation, the identification of the major constituents of a sample can usually be done with a high degree of confidence, but, when minor or trace level elements are considered, errors can arise unless careful attention is paid to the problems of spectral interferences, artifacts, and the multiplicity of spectral lines observed for each element. Because of the differences in approach to qualitative EDS and WDS analysis, these techniques will be treated separately.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

7. Quantitative X-Ray Microanalysis

Abstract
With the EPMA and the SEM one can obtain quantitative analyses of ~1-µ3 regions of bulk samples using a nondestructive x-ray technique. For samples in the form of thin foils and sections of organic material, the size of the analyzed microvolume is reduced to about one tenth of the value for bulk samples. For metals and alloys the ZAF technique is usually employed. Pure element or alloy standards can be used and the surfaces of the samples and standards must be properly prepared and analyzed under identical operating conditions. For geological samples the a factor or empirical technique is usually employed. For this class of samples secondary x-ray fluorescence is usually not significant and oxide standards of similar atomic number as the sample are used. Biological samples are often adversely affected by the impinging electron beam. It is important to ensure that the standards are in the same form and matrix as the specimen. The purpose of this chapter is to describe in some detail the various methods by which quantitative analyses can be obtained for inorganic, metallic, and biological samples in the form of bulk specimens, small particles, thin films, sections, and fractured surfaces.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

8. Practical Techniques of X-Ray Analysis

Abstract
It has already been shown in Chapter 6 that qualitative analysis is based on the ability of a spectrometer system to measure characteristic line energies and relate those energies to the presence of specific elements. This process is relatively straightforward if (1) the spectrometer system is properly calibrated, (2) the operating conditions are adequate to give sufficient x-ray counts so that a given peak can be easily distinguished from the corresponding background level, and (3) no serious peak overlaps are present.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

9. Materials Specimen Preparation for SEM and X-Ray Microanalysis

Abstract
One of the great strengths of scanning electron microscopy is the fact that many specimens can be examined with virtually no specimen preparation. Specimen thickness is not a consideration as is the case in transmission electron microscopy. Therefore, bulk specimens can be examined in the SEM with a size limited only by considerations of accommodation in the specimen stage. For the examination of images of topography contrast from metal and ceramic specimens, the only specimen preparation which is necessary is to ensure that the specimen is thoroughly degreased so as to avoid hydrocarbon contamination and, in the case of insulators, to provide a conductive coating. Techniques for cleaning surfaces include solvent cleaning and degreasing in an ultrasonic cleaner, mechanical brushing, replica stripping, and chemical etching. These techniques should be used starting with the least damaging and employing only the minimum cleaning necessary. Usually the first step is to use a solvent wash such as acetone, toluene, or alcohol in an ultrasonic cleaner. Several specific techniques for cleaning metal surfaces are described by Dahlberg (1976).
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

10. Coating Techniques for SEM and Microanalysis

Abstract
Nearly all nonconductive specimens examined in the scanning electron microscope or analyzed in an electron probe microanalyzer need to be coated with a thin film of conducting material. This coating is necessary to eliminate or reduce the electric charge which builds up rapidly in a nonconducting specimen when scanned by a beam of high-energy electrons. Figures 10.1a and 10.1b show examples of pronounced and minor charging as observed in the SEM. In the absence of a coating layer, nonconductive specimens examined at optimal instrumental parameters invariably exhibit charging phenomena which result in image distortion and thermal and radiation damage which can lead to a significant loss of material from the specimen. In extreme situations the specimen may acquire a sufficiently high charge to decelerate the primary beam and the specimen may act as an electron mirror. Numerous alternatives to coating have been proposed and some of these will be discussed in this chapter. Much of what will be discussed is directed towards biological material and organic samples simply because these types of specimens are invariably poor conductors and more readily damaged by the electron beam than most inorganic materials. However, it is safe to assume that the methods which will be described for organic samples will be equally effective for nonconducting inorganic specimens.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

11. Preparation of Biological Samples for Scanning Electron Microscopy

Abstract
In this and the next chapter we will consider the practical aspects of specimen preparation for both the scanning electron microscope and the x-ray microanalyzer. Although the two types of instrument are very similar and in many respects can be used interchangeably, it is useful, from the biologist’s point of view, to consider the preparative techniques separately. The scanning electron microscope gives morphological information, whereas the x-ray microanalyzer gives analytical information about the specimen. It is important for the user to appreciate fully these differences as they have a significant bearing on the rationale behind the specimen preparation techniques. The methods and techniques which are given in these two chapters will provide the optimal specimen preparation conditions for scanning microscopy or x-ray microanalysis. It must be realized that anything less than the optimal conditions will result in a diminished information transfer from the specimen. It will also become apparent that it will be frequently necessary to make some sort of compromise between the two approaches, which must result in less information from the specimen.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

12. Preparation of Biological Samples for X-Ray Microanalysis

Abstract
Any preparative technique for x-ray microanalysis must result in reasonably recognizable structural detail, while at the same time preserving in situ the elemental constituents one wishes to analyze. This is frequently difficult, and some elaborate and ingenious preparative techniques have been devised to achieve this goal. In many respects the principles of the preparative techniques for x-ray microanalysis are not dissimilar to those used in scanning electron microscopy. The important difference is that the methods of preparation, while seeking to preserve ultrastructure, should not achieve this goal at the expense of the soluble cell constituents. In the discussions that follow it is not intended to provide innumerable recipes which could be applied to specific biological systems, but rather to consider the principles underlying the methodology and thus allow experimenters to devise their own methods for the particular sample in question. The subject is not new, and while an attempt will be made to give details of some of the more recent advances, reference should be made to the earlier studies and collected papers by Hall, Echlin, and Kaufmann (1974), Echlin and Galle (1976), Chandler (1977), Echlin and Kaufmann (1978), Morgan et al. (1978), and Lechene and Warner (1979), which provide recipes for specific biological samples.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

13. Application of the SEM and EPMA to Solid Samples and Biological Materials

Abstract
Several examples of the use of the SEM and x-ray microanalysis are given in this chapter. These case studies were chosen to illustrate various types of SEM and x-ray microanalysis techniques that can be applied to problems of interest to the user. No attempt was made, however, to cover the wide spectrum of applications available in the literature.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

14. Data Base

Abstract
In order to calculate basic range equations (Chapter 3) and various quantitative x-ray correction schemes (Chapters 7 and 8), the analyst must have at hand the necessary input parameters. This chapter contains various tables (listed below) which include such data as atomic number, atomic weight, density, common oxides (atomic and weight fraction), mass absorption coefficients, and characteristic x-ray line energies. Also included are the J, ω, and R factors for quantitative ZAF corrections. To aid in the selection of coating materials, a compilation of important properties of selected elements is included.
Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, Charles Fiori, Eric Lifshin

Backmatter

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